Operating Systems Sockets
Operating Systems
Sockets
Outline
Socket basics TCP sockets Socket details Socket options Final notes Project 3
Socket Basics An end-point for a IP network connection
– what the application layer “plugs into”– programmer cares about Application
Programming Interface (API) End-point determined by two things:
– Host address (IP address) - name of machine– Port number - location of process
Two end-points determine a connection: socket pair– ex: 206.62.226.35,p21 + 198.69.10.2,p1500– ex: 206.62.226.35,p21 + 198.69.10.2,p1499
Ports Numbers (vary in BSD, Solaris):
– 0-1023 “reserved”, must be root– 1024 - 5000 “ephemeral”– however, many systems allow > 5000 ports
(50,000 is correct number)
/etc/services:ftp 21/tcp
telnet 23/tcp
finger 79/tcp
snmp 161/udp
Sockets and the OS
User
Socket
Operating System
(Transport Layer)
User sees “descriptor”, integer index– like: FILE *, or file index– returned by socket() call (more later)
Network Communication UDP: User Datagram Protocol
– no acknowledgements– no retransmissions– out of order, duplicate possible– connectionless
TCP: Transmission Control Protocol– reliable (in order, all arrive, no duplicates)– flow control– connection– duplex– (Project 3 uses TCP)
Socket Details
Unix Network Programming, W. Richard Stevens, 2nd edition, 1998, Prentice Hall
Socket address structure TCP client-server Misc stuff
– setsockopt(), getsockopt()– fcntl()
Addresses and Sockets
Structure to hold address information Functions pass address from app to OS
– bind()– connect()– sendto()
Functions pass address from OS to app– accept()– recvfrom()
Socket Address Structure
struct in_addr {
in_addr_t s_addr; /* 32-bit IPv4 addresses */
};
struct sock_addr_in {
unit8_t sin_len; /* length of structure */
sa_family_t sin_family; /* AF_INET */
in_port_t sin_port; /* TCP/UDP Port num */
struct in_addr sin_addr; /* IPv4 address */
char sin_zero[8]; /* unused */
}
Are also “generic” and “IPv6” socket structures
TCP Client-Serversocket()
bind()
listen()
accept()
Server
socket()
connect()
send()
recv()
Client
(Block until connection)“Handshake”
recv()
send()
Data (request)
Data (reply)
close()End-of-Filerecv()
close()
“well-known”
port
Server Functionssocket()
bind()
listen()
accept()
(Block until connection)“Handshake”
recv()
send()
Data (request)
Data (reply)
End-of-Filerecv()
close()
“well-known”
port
socket()int socket(int family, int type, int protocol);
Create a socket, giving access to transport layer service.
family is one of– AF_INET (IPv4), AF_INET6 (IPv6), AF_LOCAL (local Unix),– AF_ROUTE (access to routing tables), AF_KEY (new, for encryption)
type is one of– SOCK_STREAM (TCP), SOCK_DGRAM (UDP)– SOCK_RAW (for special IP packets, PING, etc. Must be root)
setuid bit (-rws--x--x root 1997 /sbin/ping*)
protocol is 0 (used for some raw socket options) upon success returns socket descriptor
– similar to a file descriptor or semaphore id– returns -1 if failure
bind()
sockfd is socket descriptor from socket() myaddr is a pointer to address struct with:
– port number and IP address– if port is 0, then host will pick ephemeral port
not usually for server (exception RPC port-map)
– IP address != INADDR_ANY (multiple nics) addrlen is length of structure returns 0 if ok, -1 on error
– EADDRINUSE (“Address already in use”)
int bind(int sockfd, const struct sockaddr *myaddr,
socklen_t addrlen);
Assign a local protocol address (“name”) to a socket.
listen()
sockfd is socket descriptor from socket() backlog is maximum number of incomplete
connections– historically 5– rarely above 15 on a even moderate web server!
Sockets default to active (for client)– change to passive to OS will accept connection
int listen(int sockfd, int backlog);
Change socket state for TCP server.
accept()
sockfd is socket descriptor from socket() cliaddr and addrlen return protocol address
from client returns brand new descriptor, created by OS if used with fork(), can create
concurrent server (more later)
int accept(int sockfd, struct sockaddr cliaddr,
socklen_t *addrlen);
Return next completed connection.
close()
sockfd is socket descriptor from socket() closes socket for reading/writing
– returns (doesn’t block)– attempts to send any unsent data– socket option SO_LINGER
block until data sent or discard any remaining data
– -1 if error
int close(int sockfd);
Close socket for use.
Client functions
socket()
connect()
send()
recv()
Client
“Handshake”
Data (request)
Data (reply)
close()End-of-File
connect()
sockfd is socket descriptor from socket() servaddr is a pointer to a structure with:
– port number and IP address– must be specified (unlike bind())
addrlen is length of structure client doesn’t need bind()
– OS will pick ephemeral port returns socket descriptor if ok, -1 on error
int connect(int sockfd, const struct sockaddr
*servaddr, socklen_t addrlen);
Connect to server.
Sending and Receiving
int recv(int sockfd, void *buff, size_t mbytes, int flags);
int send(int sockfd, void *buff, size_t mbytes, int flags);
Same as read() and write() but for flags– MSG_DONTWAIT (this send non-blocking)
– MSG_OOB (out of band data, 1 byte sent ahead)
– MSG_PEEK (look, but don’t remove)
– MSG_WAITALL (don’t give me less than max)
– MSG_DONTROUTE (bypass routing table)
Socket Options setsockopt(), getsockopt() SO_LINGER
– upon close, discard data or block until sent SO_RCVBUF, SO_SNDBUF
– change buffer sizes– for TCP is “pipeline”, for UDP is “discard”
SO_RCVLOWAT, SO_SNDLOWAT– how much data before “readable” via select()
SO_RCVTIMEO, SO_SNDTIMEO– timeouts
Socket Options (TCP)
TCP_KEEPALIVE– idle time before close (2 hours, default)
TCP_MAXRT– set timeout value
TCP_NODELAY– disable Nagle Algorithm
fcntl() ‘File control’ but used for sockets, too Signal driven sockets Set socket owner Get socket owner Set socket non-blockingflags = fcntl(sockfd, F_GETFL, 0);
flags |= O_NONBLOCK;
fcntl(sockfd, F_SETFL, flags); Beware not getting flags before setting! (Should not need for project 3)
Concurrent Servers
Close sock in child, newsock in parent Reference count for socket descriptor
Text segment
sock = socket()
/* setup socket */
while (1) {
newsock = accept(sock)
fork()
if child
read(newsock)
until exit
}
Parent
int sock;
int newsock;
Child
int sock;
int newsock;
Project 3: Macro Shell
Distributed Shell Client/Server Non-interactive
– command line args– get-opt.c
Uses TCP sockets– listen.c and talk.c
Security– password
Server Client(1) connect
Server
(2) ls(3) fork()and exec()
(4) data